A conventional information recording and reproducing device, for example, an SNOM (Scanning Near-Field Optical Microscope), which is described in J. Vac. Sci. technol. B13 (1995) 2813 by S. Hosaka et al., will be explained. FIG. 9 is a cross sectional view showing a structure of a conventional information recording and reproducing device. In FIG. 9, an optical fiber comprises a core 25 having a high refractive index and a clad 26 having a low refractive index surrounding the core. The tip of the cone 25 is sharpened in a conical shape. The clad 26 and a reflective film 27 are formed in a manner in which they surround the sharpened cone. The tip of this cone part faces to a recording film 14 provided on a disc substrate 13 with a small gap .delta..
The gap .delta. is 50 nm or less. The diameter of the tip of the cone part is about 0.1 .mu.m. When light from aradiation source enters the optical fiber, an evanescent light 29 leaks out from the tip of the cone part. Since the tip is adjacent to the recording film 14, if the recording film 14 is a phase changeable film, a signal mark 19 having a reflectance that is different from that of surroundings can be formed on the recording film by the thermal energy of the evanescent 29. Moreover, the signal marks 19 can be confirmed by detecting a reflected scattered light 30 from the recording film 14 by a photo detector 31.
Such an information recording and reproducing device enables to record and reproduce signals corresponding to the core's diameter of the tip. Thus, if the diameter of the signal mark 19 is 0.1 .mu.m, an information recording and reproducing device having at least 9 times higher density than a digital video disc (DVD) (the diameter of the signal mark is about 0.3 to 0.4 .mu.m) can be provided.
The conventional information recording and reproducing device has problems, for example, those shown in FIG. 10A and FIG. 10B. FIG. 10A shows a main part of the conventional information recording and reproducing device. FIG. 10B is a graph showing a distribution of light energy along the direction of light propagation (-z axis) of the device.
In FIG. 10A, a guided light 28 propagates in the optical fiber without substantial loss until it reaches the cone part (z&gt;a). In the cone part (a&gt;z&gt;0), however, an incident light 28a is divided into a refracted light 28b and a reflected light 28c, and further the reflected light 28c is divided into a refracted light and a reflected light. The reflected light divided repeatedly along the length, that is a, of the cone part. Consequently, the evanescent light 29 radiated from the tip of the cone part is extremely weakened.
In general, if the light energy of the guide light before the cone part is 1, the light energy .eta. of the evanescent light 29 released from the tip of the cone part is said to be about 10.sup.-7 to 10.sup.-6. As shown in FIG. 10B, the coefficiency of light utilization is extremely bad. Therefore, it takes a long time for recording on the recording film 14. Thus, the conventional device has a great shortcoming in terms of information transmitting rate, though the high density recording can be achieved.
Moreover, the tip (head) of the cone part is like a dot floating in space. Therefore, heat is liable to converge on the head and deterioration due to a thermal damage can readily occur. Furthermore, in order to stably scan the head while bringing it closer to the recording face 14, a complex mechanism is required. Furthermore, since the track on the recording film cannot be recognized, it is difficult to realize high density in the track direction.
In addition, in the reproducing principle using the reflected scattered light 30, only a small part of the reflected scattered light extending broadly in space is detected. Therefore, reproducing sensitivity is extremely poor.